The present invention relates to a circuit for quantizing a received signal in time at the signal receiving side of an image transmission system such as a facsimile system. More particularly, the invention relates to a quantization circuit in which the received signal can be correctly quantized even if it has been subjected to distortion such as by fluctuations in time caused, for instance, by transmission circuit difficulties.
Heretofore, in a facsimile system or the like, received signals demodulated by the demodulator were processed in a digital mode with the received signals, which are inputted continuously, being quantized by sampling at equal intervals during every scanning line. In these systems, the sampling interval t is: EQU t=(T-T'/n)
where n is the number of bits provided per scanning line, T is the scanning line transmission period, and T' is the period during which other than image data is received such as is used for synchronization.
If a signal quantized with a period t as shown in FIG. 1A is transmitted by the signal transmission side and the signal thus quantized is transmitted under the condition that no distortion is caused by the transmission system which includes the transmission lines and MODEMs (modulator and demodulator), then the signal can always be received correctly when sampling is carried out at the signal receiving side with the sampling timing shifted from the transition times of the signal at the signal transmission side by t/2 as shown in FIG. 1B.
However, if the received signal has been subjected to distortion such as by fluctuating in the time position of the signal caused by transmission circuit problems as illustrated in FIG. 1C, then in the conventional system with the fixed sampling interval t, the distortion affects the sampling result at the signal receiving side. That is, jittering is caused in the recorded image. Thus, the conventional system suffers from a drawback that the recovered image can be considerably low in quality.
Accordingly, an object of the invention is to provide a system for quantizing received signals in which all of the above-described difficulties accompanying a conventional system have been eliminated and received signals can be quantized correctly even if they have undergone distortion such as by fluctuations in their time position so that degradation of the received images is prevented.